1. Field of The Invention
This invention relates to fluids for use in boring and trenching operations in the foundation drilling industry, the subterranean construction industry, and in well drilling. More specifically, this invention relates to earth support fluids, their composition, and techniques for preparing, using, and maintaining them.
2. Description of the Prior Art
In creating foundations and shorings for buildings, bridges and other structures and in excavating and subterranean boring for installation of utilities, barrier walls, transit ways and drainage systems, fluids have been used to support the surrounding earth during construction operations. Whenever subterranean construction must be accomplished in granular, unstable, water-saturated or gas-charged soils, it has been customary to fill the boreholes, tunnel faces or excavations temporarily during construction with water-based earth-support fluids, also known as slurries or muds. These slurries have consisted essentially of water, a thickening agent and earth solids from the excavation.
The conventional materials for making these slurries are clays such as bentonite and attapulgite. More recently, water-soluble polymers have been introduced and used in place of or in combination with such clays. The most widely used type of water-soluble polymer in these applications is a partially-hydrolyzed polyacrylamide (PHPA), in the form of a primarily linear, or non-crosslinked, long chain polymer with an anionic charge density of twenty to thirty mole weight. This chemistry is available in both liquid water-in-oil emulsion form and in dry form. Other polymers include dry forms of guar gum, xanthan gum, cellulosic polymers, and blends of these. All of these polymers, including the PHPAs, have been optimized to be completely water soluble and/or homogenizable.
These polymers are designed to mix without forming masses or pearls of undissolved or semi-hydrated polymer. It has been customary to prehydrate and maximize solubilization and homogenization of these materials before introduction of the fluid into the excavation or borehole. This has been accomplished through the use of induction systems, recirculation, agitation, and processing of the polymer, and retaining the prepared fluid in a maturation tank for a period of time to maximize solubilization and homogenization of the constituents of the fluid, prior to introduction of the fluid into the excavation or borehole.
Clay slurries or muds are formulated with about five to ten percent bentonite in fresh water or about five to ten percent attapulgite in salty water. When traditional polymers are used, the dosage is generally much lower and ranges from about 0.01 to about 3.0 percent for most applications. Whether the slurries are formulated with clays or polymers, the object is to create a viscous fluid that stabilizes and supports the walls of the excavation, excludes groundwater and gases from the excavation, and facilitates the progress of the construction project. A key to success in these efforts is to avoid loss or seepage of the excavating fluid into the surrounding earth during the excavating operation. If the fluid is lost into the earth (e.g. into a sandy formation) and the excavation cannot be kept full of fluid, the excavation can collapse and groundwater or gases can enter the excavation. Excessive fluid loss can also disrupt naturally occurring cohesive forces between the formation solids. Disruption of this natural cohesion destabilizes the formation.
In the prior art the viscosity of the earth support slurries has been maintained by design generally in a range of about 30 to about 45 seconds per quart as measured with a Marsh Funnel according to viscosity measurement procedures standardized by the American Petroleum Institute. This range of viscosities was felt, in light of "industry knowledge" of the prior art, to be the most effective and least damaging.
The cohesion of granular earth solids is governed by the earth binding coefficient of the slurry. The earth binding coefficient is the composition's affinity for earth solids, which causes the earth support fluid to chemically and physically bond or attach to excavated earth, both on the excavation tool and on the excavation walls. This property preserves or improves the tendency of grains of earth to hold together in mass as opposed to separating into individual grains or smaller masses. This property also improves the low-shear adhesion of masses of granular earth solids to excavating tools, which aids in conveying of the earth solids up from the excavation. The earth binding capacity is also manifested as the ability of the earth support fluid to film or encapsulate clay bearing mineral solids and thereby reduce their tendency to adsorb, absorb or take up water.
When clays or dry-form conventional polymers are used to prepare the slurries, specialized equipment and procedures have been necessary to mix the clay or polymer (powder or granules) into the water, stir and homogenize the mixture, transfer the slurry between the boreholes or excavations and the mixing equipment, and process the slurry to remove sand and excavated solids. This equipment normally includes large mixing and holding tanks, agitators, pumps, hoses or pipes, cyclonic desanders and vibrating screen machines. Such equipment is large, relatively complex and costly.
With some polymers, it has been possible to simplify the mixing and handling equipment to reduce the investment required and to simplify the handling of the slurries. Depending on slurry composition and properties, it is sometimes possible to operate without desanding equipment and related tankage. For example, this might be possible when nonthixotropic polymer slurries are in use. However, all dry-form polymers in the prior art have required some type of specialized equipment for successful prehydration and mixing of the polymer into a homogeneous and fully-dissolved form. The practice of using such mixing or prewetting equipment is referred to herein as "indirect addition."
When the conventional liquid emulsion PHPA (mentioned above) is used as the primary slurry additive, it has occasionally been added directly into the borehole or excavation, and the drilling or excavating tools have been used to mix it with water and/or fluid in the borehole. However, this is not the industry-preferred method for emulsion PHPA addition. Typically an eduction unit and hydration tank with recirculating pumps are utilized to insure complete solubilization of the polymer prior to introduction to the excavation, or borehole.
Direct addition mixing has been claimed to be possible for the liquid form of PHPAs, which has historically been considered superior for mixability with, and solubility in, water when compared to dry polymers, in a number of industries. Emulsion polymers have been promoted as possessing less tendency toward agglomeration and wastage of polymer than can occur when attempting to mix dry-form polymers with less-than-adequate equipment. Insufficient shear is generally applied to accomplish complete inversion of the emulsion PHPA and dispersion of the polymer in direct-addition applications. This has in actuality caused considerable wastage of added polymer due to incomplete inversion or inactivation of the polymer in the prior art.
The various industries have not widely recognized or addressed the problems of incomplete inversion and inactivation of liquid emulsion PHPA. Some manufacturers and consumers have documented hydration and solubilization problems occurring due to the hydrophobic surfactants and the mineral oil contained in the emulsion polymers. Problems such as delays in polymer inversion due to inadequate inversion systems in the emulsions, and inadequate mixing shear to fully break the emulsion micelle and develop the polymer chains from an emulsion package have also been noted. It has also been noted that the surfactant and oil contained in the emulsion did not completely disassociate from the emulsion-based PHPA. This reduces performance efficacy through the coating and/or blinding of active polymer sites. Some industries have recommended the use of a high shear pump, such as the Echols pump, or centrifugal pump, for premixing of both emulsion and dry form PHPAs to insure complete homogenization and solubilization. One publication specifically addressing these problems is "Field Application of PHPA Muds" by A. G. Kadaster & G. J. Guild, Amoco Production Co.; G. L. Hanni, Amoco Norway Oil Co.; and D. D. Schmidt, Amoco Production Co., SPE presentation, 1989, San Antonio, Tex.
The ability to rapidly mix and yield polymer directly in the borehole or excavation is advantageous because it eliminates the need for costly, cumbersome mixing and processing equipment. It can significantly reduce time required to drill, excavate and construct piers, walls, pads, wells, etc.
Whenever polymer has been used, a primary objective in mixing polymer into water or earth excavation fluid has been to create a homogeneous solution or mixture and to accomplish complete dispersion and dissolution of the polymer as readily as possible. Completely dissolving and homogenizing the polymer in the water or fluid has been considered a key to optimum performance. Incompletely homogenized polymer of any kind, whether in the form of agglomerates, polymer strings, "fisheyes," gels, microgels, pearls or masses has been seen as disadvantageous and wasteful. Avoiding the presence of incompletely hydrated polymer in the slurry has been a prime objective of fluid design and mixing practice.
Although bentonite is the principal material used for preparing slurries, bentonite slurries have become increasingly regulated as pollutants, and as a result, disposal costs have risen. Bentonite slurries must now generally be removed from a construction or drilling site and disposed of in a designated landfill or in accordance with local authorities and permits. This additional cost, along with the high capital cost and complexity of bentonite slurry mixing and processing equipment has prompted increased use in subterranean construction and drilling industries of polymer, especially the liquid emulsion PHPA.
According to current "industry knowledge", emulsion PHPA requires less equipment to process and is seen as less polluting. However, emulsion PHPA contains refined hydrocarbon oils and surfactants, and thus creates environmental pollution problems of a different kind. This oil and surfactant-pollution problem has only recently been acknowledged or widely considered. The EPA and other regulatory agencies are beginning to recognize the significant toxicity of these hydrocarbons in oil and gas drilling. In addition to liberating a hydrocarbon into the environment, PHPA emulsion is beginning to be recognized as a potential fire hazard on-site. Special fencing and precautions are now required on sites where emulsion polymer is being stored.
There has been little understanding of, or remedy offered for a problem frequently encountered in boring, drilling, and trenching (in non-mineral based slurries)--the problem of earth support fluid loss into permeable soil formations. This seepage or "fluid loss" is common in granular, permeable soils, such as sand and gravel, and in fractured and fissured formations. Fluid loss can seriously interfere with the processes of excavation, drilling, or construction. Excessive fluid loss is a primary cause of destabilization of the excavation, pollution of groundwater, delays in excavation and boring projects, increased concerns for safety, and increased consumption of slurry, slurry additives, concrete, cement, grout, etc.
Extreme cases of fluid loss have been attacked by dumping bentonite, silts, and/or other available colloids into the excavation or by boring native silts and clays in an attempt to form a mineral-enhanced filter cake at the formation interface of the excavation. When a mineral-based or mineral supplemented slurry is used in fine-grain sands, the dispersed mineral colloids in the slurry can provide improved control of fluid loss because the pores in the soil are small. But mineral-based and mineral supplemented slurries, due to the thick filter cakes they create, reduce borehole gauge. This reduced gauge can reduce the diameter of formed structures or casings created in the excavations and boreholes. Similarly, mineral-based and mineral supplemented filter cakes can negatively affect the geometry of the formed structures or casing. Additionally, mineral-based or mineral supplemented filter cakes, as a sheath of continually reactive and hydratable colloids at the interface between the concrete and surrounding earth, can reduce skin friction on which formed or poured structures rely for their load-bearing capacities. Reduced friction may promote instability, movement and stress on these structures, which can damage the subterranean structure and the super-structure that rests on them.
With polymer-based slurries containing no bentonite or other cake-building inorganic colloids, fluid loss control has been unattainable or poorly realized. The dissolved water-soluble polymers cannot plug the pores in the granular soil or create a filter cake as can bentonite and inorganic colloids. It has been impossible to control fluid loss without adding mineral colloids, similar colloids or finely-divided materials such as native clays and silts incorporated into the slurry from the excavation.